More likely to be the large explosion sound, 4, which is called all over for blown up stuff...
IF we want to narrow this down then
useful information is really required, more than "this stuff happens sometimes".
e.g. - what precisely was firing at whom with what sort of weapon and ammo and did you hit or miss it? If you hit it, what hit message was displayed?.
"My Tiger 2 fired at a T-70 with HEAT and the big boom occurred but nothing happened" might narrow things down a bit.
At least then we could set up a test scenario with tank A and target b and blaze away till we get the required result(or not..
).